1. Field of the Invention
The present invention relates to method of three-dimensional (3D) image reconstruction and transmission electron microscope for obtaining three-dimensional images of specimens.
2. Description of Related Art
Today, transmission electron microscopes are receiving attention as apparatus capable of obtaining three-dimensional (3D) images of specimens. In such a transmission electron microscope equipped with a function of reconstructing a 3D image of a specimen, the specimen is tilted. TEM images are gained at various tilt angles of the specimen, and a CT (computerized tomography) method is applied to the gained TEM images to reconstruct a 3D image of the specimen.
It is known that in order to have a TEM image with good contrast, it is better to slightly shift the focal point of the objective lens from the positive focal point lying on the specimen. Accordingly, in a transmission electron microscope having the prior art function of reconstructing 3D images, to image a TEM image with good contrast at each tilt angle of specimen, the following process steps (1) and (2) have been performed at each tilt angle.
(1) The position of the positive focal point of the objective lens is detected.
(2) The position of the focus of the objective lens is set to a position deviating from the position of the positive focal point by a fixed amount of defocus Δf. At this set focal point, a TEM image is taken.
The process steps (1) and (2) above are described, for example, in Japanese Patent Laid-Open No. H4-104445. A transmission electron microscope for obtaining 3D images is described, for example, in Japanese Patent Laid-Open No. H4-337236.
It is considered that the amount of defocus Δf (the amount of deviation of the focus from the positive focal point) occurring during imaging greatly affects the image quality of the obtained TEM image. Especially, where the specimen is tilted, it is considered that the optimum amount of defocus varies depending on the tilt angle because the apparent thickness of the specimen varies. It is also considered that the optimum amount of defocus is affected by the material of the specimen.
In the prior art, however, the amount of defocus Δf is fixed. Even if the specimen is tilted, the same amount of focus Δf is used.